This invention relates to a photometer tube for a microscope. Such tubes, as known up to the present time, may be classified into two types.
Tubes of the first type are those in which, in order to visualize the measuring field stop, the entire observation ray path is conducted through what may be called a bypass loop and is reflected at an obliquely positioned measuring field stop, which is usually developed as a concave mirror. Such instruments are described, for example, in Federal Republic of Germany patent document No. 24 06 415 and its corresponding U.S. Pat. No. 3,887,283 of Merstallinger et al., granted June 3, 1975, and in Fed. Rep. Germany patent document No. 32 03 142 and its corresponding U.S. Pat. No. 4,518,230 of Weber, granted May 21, 1985, and in Fed. Rep. Germany patent document No. 32 13 145 and its corresponding U.S. Pat. No. 4,568,188 of Weber et al., granted Feb. 4, 1986.
In instruments of this first type, it is difficult to make the measuring field stop variable, and difficult to illuminate it. Moreover, instruments with bypass loops have the disadvantage that, because of the length of the observation ray path, there comes into being a post-magnification factor, which results in a darker image. This interferes particularly with the observation of objects which fluoresce with low intensity.
Referring again to the above mentioned two types of instruments, in those of the second type the observation ray path is deflected by a beam splitter directly into the eyepiece tube. The image of a back-illuminated measuring field stop is superimposed on an intermediate image produced in the eyepiece tube because of the fact that the auxiliary ray path for the imaging of the measuring field stop is led coaxially backward to the photometry ray path and, with the help of a retroreflector arranged behind the second outlet of the beam-splitter prism, is reflected into the eyepiece tube. Instruments of this type are described, for example, in Feb. Rep. Germany patent No. 1,215,954 of Weber, granted Nov. 17, 1966, and in Swiss patent No. 615,762 and its corresponding British patent No. 1,582,346 of Leitz, published Jan. 7, 1981, and in Fed. Rep. Germany patent document No. 34 43 728 and its corresponding European patent application No. 85308145.3 of Schindl, published in English on June 4, 1986, as publication No. 0 183 416.
Instruments of this second type have the possibility of making the measuring field stop interchangeable, and of illuminating it in a simple way. However, they also have the disadvantage that the light required for illuminating the measuring field stop gives rise to reflections in the image space, that is, on the tube lens, on the objective, or on other optical elements in the viewing ray path. These reflections are disturbingly superimposed on the image of the object.
Fed. Rep. Germany Auslegeschrift No. 1,813,499 of Voss, published Oct. 11, 1973, discloses a photometer tube in which the viewing ray path is reflected directly into the eyepiece by means of a first beam splitter, and in which the light passing through the measuring field stop is led around a bypass loop and, by means of a second beam splitter, is again superimposed on the viewing ray path. A disadvantage of this solution is that it supplies only a rather dim image of the measuring field stop, because of the arrangement of several beam splitters, one after another, through which the light must travel.
It is the object of the present invention to provide an improved photometer tube for a microscope, which will supply an intermediate image with the measuring field stop visible therein with the greatest brightness possible and with the greatest possible freedom from undesirable reflections.